Imaging optics for cameras available for this decade, such as monitoring cameras generally called CCTV and high resolution digital cameras of mega pixels, for example, of 2 million or even higher, often encounter an undesirable phenomenon figured as chromatic blurring where the feature of high resolution results in residual lateral chromatic aberration being developed conspicuously in an image. Especially, in leaving an aperture stop loosened to let light beams come in and get an image of a small but brightly irradiating object in a dark background, for instance, a blue chromatic blur gets conspicuous surrounding such a brightly irradiating object in the resultant image.
“Larger aperture”, “downsizing”, and “reduced cost” are universal goals continually pursued, and it is desired to use a reduced number of pieces of lens and/or to avoid a use of special low-dispersion glass material.
A compound lens of a glass lens having its surface laid with resin layer to serve as optics in the prior art has come to be broadly used for optics of cameras and office electronic machines like a copying machine. With such a compound lens of the glass lens coated with resin layer, the resin layer is defined, circumferentially departing from a valid range of the resin layer that works for the practical imaging, in order to attain the desired figure tolerance within the valid range of the resin layer suitable as the optics.
Specifically, the resin layer, when precisely defined, unavoidably has some part with a defect, such as figure dullness, that fails to attain the required figure tolerance as the optics, and for the reason, the resin layer spreads considerably beyond the valid range although useless for the imaging in practice. The glass lens itself, laminated with the resin layer, must have a greater outer diameter beyond the range the resin layer occupies. This means the existence of an optical surface area outside the valid range that does not contribute to the imaging, which brings about flare.
Rays of incident light, after passing the periphery of the compound lens with the resin layer, are refracted differently from the direction in which a primary valid light flux contributing to the imaging is done, and resultantly, this may cause flare light in the imaging plane such as a film surface or a CCD surface. As with the optics incorporating such a compound lens having the resin layer thereon, the inventors find a necessity of preventing the flare resulted from the part other than the valid range in the compound lens.
With a lens but the compound lens coated with the resin layer thereover, a light shielding member, which serves also as a lens holder, may be provided almost reaching the boarder of the valid range, so as to block the undesired light that may cause the above-mentioned flare. Such a compound lens, however, is so soft in a surface of the resin layer to affect the figure precision within the valid range, and therefore, the means of providing the light shielding member is not suitable. Another means of coating or optical blacking cannot be alternative because of an almost mirror-like surface of the resin layer.
In the light of the above-mentioned circumstances, alternatively disclosed is a compound lens that is comprised of a glass lens, a resin layer coating the glass lens on its surface to turn the simple glass lens to a compound lens, and a light shielding member of low transmissivity to visible light in position between the glass lens and the resin layer, departing from a valid range in the periphery of the lens apart from the optical axis (e.g., see Patent Document 1 listed below). By virtue of such a compound lens, flare caused by a light flux passing outside the valid range can be effectively obviated.
In addition to that, an optical aperture stop useful to avoid residual aberration in the optics is disclosed in Patent Document 2 listed below. This type of a lens architecture should have optics requirements, in advance, of a relatively low brightness, a narrow field angle, and a symmetrical lens arrangement fore and after about an aperture stop. The one disclosed in Patent Document 2 preconditions a Gaussian type of the similar architecture where a filter is in the midst between fore and rear groups of lenses, having a plurality of divided areas that respectively permit light of varied wavelengths to be transmitted, and such a filter is segmented, including a center circular area and a peripheral annular band area.
This kind of technology is further disclosed in Patent Document 3 listed below as implemented in a picture optics having a lens with a light shielding film so as to reduce abaxial aberration. In a prior art as in Patent Document 4, furthermore, a projection color image display apparatus having a wavelength-selective aperture stop is set forth.
Furthermore, Patent Document 5 listed below teaches a television camera lens apparatus where coating is laid over and integrated with a fore group lens element so as to serve as a filter area shaped in a circle with a predetermined radius about the optical axis.
Patent Document 1: Japanese Patent Preliminary Publication No. H8-179105,
Patent Document 2: Japanese Patent Preliminary Publication No. H11-125849,
Patent Document 3: Japanese Patent No. 2846821,
Patent Document 4: Japanese Patent Preliminary Publication No. S57-37990, and
Patent Document 5: Japanese Patent Preliminary Publication No. H5-110938.
In the compound lens disclosed in the above-identified Patent Document 1, a development of the flare can be effectively prevented, but there is no effort to reduce chromatic aberration, especially, to compensate for chromatic aberration components of halo. The “halo” means dimensions of light dispersed in some dimensional space instead of converging to a single spot because a light flux emitted from a single spot of an object and incident upon the lens is refracted due to various aberrations such as spherical aberration.
The currently available imaging optics of relatively large aperture, downsized, and of reduced cost, which is suitable for a monitoring camera and a high-resolution digital camera, is at greater manufacturing cost because of devices of adding lens elements, and/or, using special low-dispersion glass material for reducing the chromatic aberration in halo.
The invention set forth in Patent Document 2 teaches an almost symmetrical type of the optics architecture about the centered optical aperture stop, as stated above, where simply a compensating filter is located in the position of the optical aperture stop around the midst between fore and rear groups of lenses. In general, optical lens, as it is greater in aperture stop and greater in field angle, develops the flare more in an upper portion of the incident light flux closer to its upper limit line, namely, in a portion of the incident light flux converging downward to a focusing plane, and the upper portion of the incident light flux propagates closer to the optical axis rather than its upper limit line when it passes the aperture stop in the middle of the lens elements. An essential condition for the compensation is making the upper portion of the light flux propagate in the periphery of the filter, and nevertheless, the invention in the Patent Document 2 is defective in that it cannot be applied to a lens of large field angle and of large aperture since the upper portion of the incident light flux where abaxial beams are at greater incident angles propagate hardly passes the periphery of the filter defined approximately in a doughnut shape. The upper portion of the light flux can pass the periphery of the filter if a center region of the filter is decreased, instead enlarging its periphery, but this results in the light having its greater amount cut off, which cannot be a good trade-off between the anti-flare effect and elicited demerits such as color imbalance. To avoid that, the filter must have its periphery reduced in proportion to the reduction of its center region, and the resultant filter looses its merit and works like a dark lens at most. As a consequence of overcoming this problem, however, the resultant optics can be applied only to a telescopic lens of small incident angle.
When the coated filter is incorporated in the optics, incident properties of the coating bring about varied results such as a wavelength deviation and a performance degradation, depending upon an incident angle. The filter, when located in the position as described in Patent Document 2, leads to a variation in the incident angle between axial and abaxial light fluxes, and this causes a deteriorated effect of the compensation for chromatic aberration components. When applied to a generic lens of a non-symmetrical optics architecture, the filter brings about the greater variation in the incident angle, and this causes a considerably poorer compensation for chromatic aberration components.
For these reasons, an effective and well-balanced reduction of the chromatic aberration components, especially, those in the halo, cannot be conducted, and the compensation for the chromatic aberration components is left insufficient.
The above-identified Patent Documents 3 and 4 disclose the similar.
A television camera lens apparatus disclosed in Patent Document 5 relies on a technology of creating a coating layer serving as a filter in a surface center area of a lens element in fore or rear group of lenses so as to reduce a transmissivity in the center area of the lens element. The filter position is just opposite to the present invention, and thus, it is impossible to reduce the chromatic aberration components of the halo developed in the peripheral area of the lens.
The present invention is made to overcome the aforementioned disadvantages in the prior art, and accordingly, it is an object of the present invention to provide an imaging optics applicable to a non-symmetrical optics architecture, which is needless to add a lens element and use a expensive special low-dispersion glass material to advantageously compensate especially for chromatic aberration components in comparison with the prior art imaging optics of the similar optical performances.
It is another object of the present invention to provide an imaging optics compensating for chromatic aberration components of halo by means of an improvement over a diurnal-nocturnal lens eliminating a latent defocusing in visible and near infrared wavelength ranges and over an ultraviolet rays compatible lens eliminating a latent defocusing in visible and near ultraviolet wavelength ranges both of which, even when made of expensive special low-dispersion glass material, fail in some way to compensate well for the chromatic aberration components in halo.
It is further another object of the present invention to provide an imaging optics efficiently eliminating chromatic aberration components in halo by means of a disposition of a light shielding means in an area where light flux desired to remove concentrates in the periphery of a lens system, namely behind an aperture stop in a practical embodiment.
It is still another object of the present invention to provide an imaging optics compensating well for chromatic aberration components in halo without losing a color balance in a resultant image in the event of a significantly conspicuous chromatic aberration in halo of a light flux, by means of a light shielding means capable of covering a wider wavelength range to shield as it is closer to the outermost periphery.